Film and method for manufacturing same

ABSTRACT

Disclosed are a film with high strength which does not easily deteriorate, and a method for manufacturing the film. A step for manufacturing a film formed on the molding surface of a die, includes an initial film-forming step for forming, on the molding surface of the die, an initial film having a carbon film which has a plurality of nano-carbons, and to which a plurality of fullerenes are applied, and a nitrogen compound layer and a sulfurized layer which are situated between the carbon film and the die, and an intermittent heating step for intermittently heating the initial film formed in the initial film-forming step under a non-oxidizing atmosphere.

TECHNICAL FIELD

The present invention relates to a film formed on the surface of an iron-based material, and to a method for manufacturing the film.

BACKGROUND ART

Conventionally, publicly known is a technique for forming a predetermined film on the molding surface of a die (the surface of an iron-based material) in order to accomplish decrease of mold-release resistance and the like in die casting (for example, see Patent Literature 1).

Patent Literature 1 discloses a technique for forming a film on the molding surface of a die by applying fullerenes to a carbon film having nano-carbons such as carbon nanotube.

According to Patent Literature 1, by filling apertures in the carbon film with the fullerenes to smoothen the uneven surface of the carbon film, mold-release resistance is reduced.

However, if the technique described in Patent Literature 1 is used, in the case of incomplete combination of the carbon film and the fullerenes, the fullerenes are removed by alkaline solvent, and the film deteriorates. Moreover, the film is easily peeled off because the film has insufficient strength.

Therefore, the technique described in Patent Literature 1 has room for improvement.

CITATION LIST Patent Literature

Patent Literature 1: JP 2010-36194 A

SUMMARY OF INVENTION Problem to Be Solved By the Invention

The objective of the present invention is to provide a film with high strength which does not easily deteriorate, and a method for manufacturing the film.

Means for Solving the Problem

A first aspect of the invention is a method for manufacturing a film formed on a surface of an iron-based material, including an initial film-forming step for forming, on the surface of the iron-based material, an initial film including a carbon film, a nitrogen compound layer and a sulfurized layer, the carbon film having a plurality of nano-carbons, and a plurality of fullerenes which is applied to the carbon film, the nitrogen compound layer and the sulfurized layer being situated between the carbon film and the iron-based material, and an intermittent heating step for intermittently heating the initial film formed in the initial film-forming step under a non-oxidizing atmosphere.

Preferably, the iron-based material is a die used for casting, which has a molding surface, in the initial film-forming step, the initial film is formed on the molding surface of the die, and in the intermittent heating step, an oil-based release agent is applied to the molding surface of the die on which the initial film is formed, and then the casting is performed a plurality of times using the die.

A second aspect of the invention is a film which includes a carbon film having a plurality of nano-carbons, and which is formed on a surface of an iron-based material. The film contains hard amorphous carbon, Fe₄N, Fe₃C, martensite, and Fe₃O₄. Sulfur-diffusion ratio in the carbon film is over 50%.

Preferably, hard amorphous carbon, Fe₄N, Fe₃C, martensite, and Fe₃O₄ are identified by an X-ray diffraction method, and the sulfur-diffusion ratio is found by mapping analysis with an EPMA.

EFFECTS OF THE INVENTION

The present invention makes it possible to minimize deterioration and separation of a film.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 shows a step for manufacturing a film according to an embodiment of the present invention.

FIG. 2 illustrates an initial film.

FIG. 3 illustrates the film according to an embodiment of the present invention.

FIG. 4 shows a result of mapping analysis of the initial film with an EPMA, in which FIG. 4( a) shows carbon present in the initial film, and FIG. 4( b) shows sulfur present in the initial film.

FIG. 5 shows a result of mapping analysis of the film according to an embodiment of the present invention with the EPMA, in which FIG. 5( a) shows carbon present in the film according to an embodiment of the present invention, and FIG. 5( b) shows sulfur present in the film according to an embodiment of the present invention.

FIG. 6 shows a result of mapping analysis of a comparison film with the EPMA, in which FIG. 6( a) shows carbon present in the comparison film, and FIG. 6( b) shows sulfur present in the comparison film.

DESCRIPTION OF EMBODIMENTS

With reference to FIGS. 1 to 3, described below is a step Si for manufacturing a film 1 as an embodiment of a method for manufacturing a film according to the present invention.

The film 1 is formed on the molding surface of a die used for die casting and the like.

The step S1 is a step for forming the film 1 on the molding surface of the die.

In the present embodiment, the die is an iron-based material made of alloy tool steel (JIS G4404) such as SKD61.l

As shown in FIG. 1, the step Si includes an initial film-forming step S11 and an intermittent heating step S12.

The initial film-forming step S11 is a step for forming an initial film 100 on the molding surface of the die. The initial film-forming step S11 is a conventional technique, and specifically is the step disclosed in JP 2010-36194 A. Therefore, detailed explanation of the initial film-forming step S11 is omitted.

FIG. 2 illustrates the initial film 100 produced in the initial film-forming step S11.

As shown in FIG. 2, the initial film 100 is a film produced by the conventional technique, and includes a diffusion layer 110, a nitrogen compound layer 120, a sulfurized layer 130, and a carbon film 140.

The diffusion layer 110 is a part of the die into which nitrogen is diffused, and is formed in the vicinity of the molding surface of the die.

The nitrogen compound layer 120 is a layer containing Fe₃C and a nitrogen compound such as Fe₂N or Fe₃N. The nitrogen compound layer 120 is formed on the diffusion layer 110.

The sulfurized layer 130 is a layer containing a sulfur compound such as FeS. The sulfurized layer 130 is formed on the nitrogen compound layer 120.

The carbon film 140 is a layer having nano-carbons. The carbon film 140 is situated at the outermost part (the uppermost part in FIG. 2) of the initial film 100.

The carbon film 140 has a plurality of hard amorphous carbons 141, a plurality of nano-carbons 142, and a plurality of fullerenes 143.

The hard amorphous carbon 141 is an amorphous material consisting primarily of carbon. The plurality of hard amorphous carbons 141 are scattered in roughly a position where the diffusion layer 110 and the nitrogen compound layer 120 are formed.

The nano-carbon 142 belongs to the nano-carbons such as carbon nanofiber, carbon nanotube, carbon nanocoil, and carbon nanofilament. The plurality of nano-carbons 142 are formed to extend toward the surface of the initial film 100 (extend upward in FIG. 2) from the plurality of hard amorphous carbons 141. The plurality of nano-carbons 142 extend to reach the surface of the initial film 100.

The fullerene 143 is a carbon cluster consisting of a plurality of carbon atoms, typified by C₆₀. The fullerene 143 may be one of fullerene derivatives to which a predetermined chemical modification is applied. The plurality of fullerenes 143 exist between the nano-carbons 142.

As mentioned above, the initial film 100 includes the carbon film 140 having the plurality of nano-carbons 142, to which the plurality of fullerenes 143 are applied, and the nitrogen compound layer 120 and the sulfurized layer 130 which are situated between the carbon film 140 and the die.

In the present invention, a method for manufacturing an initial film is not limited as long as the initial film includes, at least, a carbon film having nano-carbons, to which a plurality of fullerenes are applied, and a nitrogen compound layer and a sulfurized layer which are situated between the carbon film and the die.

As shown in FIG. 1, the intermittent heating step S12 is a step for intermittently heating, under a non-oxidizing atmosphere, the initial film 100 produced in the initial film-forming step S11.

In the intermittent heating step S12, the initial film 100 is intermittently heated by actually performing die casting a plurality of times using the die on which the initial film 100 is formed.

Specifically, first, an oil-based release agent such as mineral oil, synthetic oil, or vegetable oil is applied to the molding surface of the die on which the initial film 100 is formed. At this time, the oil-based release agent is applied to the molding surface of the die so that the molding surface is completely covered with the oil-based release agent. Thereby, the initial film 100 can avoid exposure to water and air, namely, can be under the non-oxidizing atmosphere.

Next, the cavity of the die is filled with a molten metal, such as aluminum alloy, with a high temperature (e.g. 600° C.), and then the die is left to stand for a predetermined period (e.g. 5 seconds). At this time, the high-temperature molten metal comes in contact with the molding surface of the die, and thereby is cooled to a predetermined temperature (e.g. 300° C.). In other words, the initial film 100 is heated at a high temperature by the molten metal immediately after supplying the molten metal to the cavity of the die, and then is cooled at the temperature of the molten metal rapidly cooled by the die.

Finally, the solidified metal (cast metal) is released from the die.

By performing the above-mentioned process predetermined times (e.g. 1000 times), the initial film 100 turns into the film 1. In other words, the film 1 is formed on the molding surface of the die.

As mentioned above, in the intermittent heating step S12, the die casting is performed a plurality of times under the non-oxidizing atmosphere using the die on which the initial film 100 is formed. Thereby, a predetermined change in temperature (heating and cooling) takes place in the initial film 100 a plurality of times. In other words, the initial film 100 is intermittently heated, which turns the initial film 100 into the film 1.

In the present embodiment, the film 1 is formed on the molding surface of the die. However, the present invention may be applied not only to the molding surface of the die but also to the surface of any iron-based material.

In the present embodiment, the oil-based release agent is applied to the molding surface every a performance of the casting. However, timing of applying the oil-based release agent to the molding surface is not limited thereto as long as the initial film 100 can be kept under the non-oxidizing atmosphere.

In the present embodiment, applying the oil-based release agent to the molding surface of the die enables the initial film 100 to be under the non-oxidizing atmosphere. However, if a method except this also enables the initial film 100 to be under the non-oxidizing atmosphere, a film according the present invention may be manufactured.

In the present embodiment, the initial film 100 is intermittently heated by actually performing the die casting the plurality of times. However, the initial film 100 may be intermittently heated by a laser, an ultrasonic wave, or the like.

FIG. 3 illustrates the film 1 produced through the step S1.

As shown in FIG. 3, the film 1 is a film produced by the conventional technique, and includes a diffusion layer 10, a nitrogen compound layer 20, a sulfurized layer 30, and a carbon film 40.

The diffusion layer 10 is the diffusion layer 110 passed through the intermittent heating step S12, and is formed in the vicinity of the molding surface of the die.

The nitrogen compound layer 20 is the nitrogen compound layer 120 passed through the intermittent heating step S12, and is formed on the diffusion layer 10.

The nitrogen compound layer 20 is a layer containing Fe₃C and a nitrogen compound such as Fe₄N. In other words, the nitrogen compound layer 20 differs from the nitrogen compound layer 120 of the initial film 100 in containing Fe₄N. When the nitrogen compound layer 120 goes through the intermittent heating step S12, Fe₂N or Fe₃N present in the nitrogen compound layer 120 of the initial film 100 turns into Fe₄N.

Fe₄N has a dense structure compared with Fe₂N or Fe₃N.

Therefore, the film 1 which includes the nitrogen compound layer 20 containing Fe₄N has one and a half times as large peel strength (pressure at which the film separates from the die) as the initial film 100 which includes the nitrogen compound layer 120 containing Fe₂N or Fe₃N.

Consequently, the present invention makes it possible to manufacture the film 1 with high strength.

The sulfurized layer 30 is the sulfurized layer 130 passed through the intermittent heating step S12, and is formed on the nitrogen compound layer 20. The sulfurized layer 30 is formed to reach the surface of the initial film 100.

The carbon film 40 is the carbon film 140 passed through the intermittent heating step S12.

In the carbon film 40, the sulfurized layer 30 is wholly formed. In other words, sulfur diffuses throughout the carbon film 40.

Thus, in the initial film 100, sulfur gathers in the root (lower part in FIG. 2) of the carbon film 140, whereas in the film 1, sulfur diffuses throughout the carbon film 40.

Therefore, the film 1 has a friction coefficient smaller than that of the initial film 100.

Consequently, the present invention makes it possible to reduce mold-release resistance of the film 1.

The carbon film 40 has a plurality of hard amorphous carbons 41, a plurality of nano-carbons 42, and a plurality of fullerenes 43.

The hard amorphous carbon 41 is the hard amorphous carbon 141 passed through the intermittent heating step S12.

The hard amorphous carbon 41 is the hard amorphous carbon 141 which is densified by a part of the plurality of fullerenes 143 made amorphous through the intermittent heating step S12. Thus, the hard amorphous carbon 41 has a structure denser than that of the hard amorphous carbon 141 of the initial film 100.

The nano-carbon 42 is the nano-carbon 142 passed through the intermittent heating step S12. The plurality of nano-carbons 42 are formed to extend toward the surface of the film 1 (extend upward in FIG. 3) from the plurality of hard amorphous carbons 41. The plurality of nano-carbons 42 extend to reach the surface of the initial film 100.

The fullerene 43 is the fullerene 143 passed through the intermittent heating step S12.

The plurality of fullerenes 43 combine with the plurality of nano-carbons 42, which makes the carbon film 40 dense.

Therefore, the plurality of fullerenes 43 are not removed by alkaline solvent. Consequently, the present invention makes it possible to prevent the film 1 from deteriorating.

Moreover, the plurality of fullerenes 43 diffuse through the nitrogen compound layer 20 and the diffusion layer 10.

As mentioned previously, in the film 1, sulfur diffuses throughout the carbon film 40. It is presumed that this is caused by permeation of fullerenes 43 into the diffusion layer 10.

Specifically, it is presumed that, when the carbon film 140 goes through the intermittent heating step S12, mutual diffusion takes place between a relatively great number of fullerenes 143 existing in the vicinity of the surface of the carbon film 140, and a relatively great amount of sulfur existing in the root of the carbon film 140, and consequently sulfur diffuses throughout the carbon film 40.

It is preferable that the lower limit of the temperature at which the initial film 100 is cooled in the intermittent heating step S12 (the temperature of the molten metal rapidly cooled when coming in contact with the molding surface of the die) is 240° C. because generally, a fullerene is easy to diffuse at 240° C. or more.

Additionally, it is preferable that the upper limit of the temperature at which the initial film 100 is heated in the intermittent heating step S12 (the initial temperature of the molten metal) is 600° C. because the carbon film 140 of the initial film 100 may deteriorate by oxidation at the temperature higher than 600° C.

Based on an example and a comparative example, with reference to FIGS. 4 to 6, described below are characteristics of a film according to the present invention.

Before carrying out processes in the example and the comparative example, mapping analysis of the initial film 100 was performed using an EPMA (Electron Probe MicroAnalyzer). A section where the mapping analysis was performed was a surface taken from the surface of the initial film 100 toward the inside thereof.

FIG. 4 shows a result of the mapping analysis of the initial film 100 with the EPMA. FIG. 4( a) shows carbon present in the initial film 100, and FIG. 4( b) shows sulfur present in the initial film 100.

Sulfur-diffusion ratio in the carbon film may be found from the result of the mapping analysis.

Note that the sulfur-diffusion ratio in the carbon film is a percentage of sulfur in the carbon film in the section where the mapping analysis with the EPMA is performed. For example, if sulfur diffuses throughout the carbon film, the sulfur-diffusion ratio is considered to be 100%.

As shown in FIGS. 4( a) and 4(b), in the carbon film 140 of the initial film 100, sulfur is not distributed wholly, and a relatively great amount of sulfur exists in the root (lower part in FIG. 4( b)). At this time, the sulfur-diffusion ratio in the carbon film 140 of the initial film 100 is considered to be 50%.

On the other hand, the initial film 100 was analyzed by an X-ray diffraction method. Thereby, it was found that the initial film 100 contained hard amorphous carbon, Fe₂N, Fe₃C, martensite, and Fe₃O₄.

EXAMPLE

First, the oil-based release agent was applied to the molding surface of the die on which the initial film 100 was formed, and thereby the initial film 100 was under the non-oxidizing atmosphere.

Next, the cavity of the die was filled with molten aluminum alloy of 600° C., and the initial film 100 was heated for 5 seconds at 4000 kcal/m² on the heat-transfer interface (the molding surface of the die) with heat-transfer coefficient of 6000 W/m²K (600° C., 50 MPa).

Finally, the solidified metal (cast metal) was released from the die.

By performing the above-mentioned process 1000 times, the film 1 was produced.

Mapping analysis of the film 1 was performed using the EPMA, similarly to the initial film 100.

FIG. 5 shows a result of the mapping analysis of the film 1 with the EPMA. FIG. 5( a) shows carbon present in the film 1, and FIG. 5( b) shows sulfur present in the film 1.

As shown in FIGS. 5( a) and 5(b), in the carbon film 40 of the film 1, sulfur is distributed wholly, and uniformly exists even in the vicinity of the surface of the film 1 (in the upper part in FIG. 5( b)). At this time, the sulfur-diffusion ratio in the carbon film 40 of the film 1 is considered to be 100%.

As mentioned above, the film 1 has the sulfur-diffusion ratio larger than that of the initial film 100. Therefore, as mentioned previously, the film 1 has the friction coefficient smaller than that of the initial film 100, and consequently has the mold-release resistance smaller than that of the initial film 100.

Note that the sulfur-diffusion ratio in the carbon film 40 of the film 1 is 100%, but is not limited thereto as long as the friction coefficient of a film according to the present invention is smaller than that of an initial film. In other words, it is only necessary that the sulfur-diffusion ratio in the carbon film of the film according to the present invention is over 50%.

On the other hand, the film 1 was analyzed by an X-ray diffraction method. Thereby, it was found that the film 1 contained hard amorphous carbon, Fe₄N, Fe₃C, martensite, and Fe₃O₄.

As mentioned previously, since the film 1 contains Fe₄N which has a structure denser than that of Fe₂N, the film 1 has strength higher than that of the initial film 100.

Comparative Example

A process similar to that of the above-mentioned example was performed except that a water-soluble release agent was used instead of the oil-based release agent, and thereby a film (hereinafter referred to as “the comparison film”) was produced.

Note that the water-soluble release agent differs from the oil-based release agent in not preventing the initial film 100 from being exposed to water and air.

In other words, the comparison film was produced by intermittently heating the initial film 100 under an oxidizing atmosphere.

Mapping analysis of the comparison film was performed using the EPMA, similarly to the film 1.

FIG. 6 shows a result of the mapping analysis of the comparison film with the EPMA. FIG. 6( a) shows carbon present in the comparison film, and FIG. 6( b) shows sulfur present in the comparison film.

As shown in FIGS. 6( a) and 6(b), in a carbon film of the comparison film, sulfur is distributed in only a fraction thereof. At this time, the sulfur-diffusion ratio in the carbon film of the comparison film is considered to be 5%.

Thus, the comparison film has the sulfur-diffusion ratio smaller than that of the initial film 100, and has a friction coefficient larger than that of the initial film 100. Therefore, the comparison film has the mold-release resistance larger than that of the initial film 100.

As mentioned above, it became clear that the film according to the present invention was produced by intermittently heating the initial film under the non-oxidizing atmosphere.

Moreover, it became clear that the film according to the present invention contained hard amorphous carbon, Fe₄N, Fe₃C, martensite, and Fe₃O₄, and that the sulfur-diffusion ratio in the carbon film was over 50%.

The mapping analysis was performed using the EPMA in order to find the sulfur-diffusion ratio in the carbon film, but a method therefore was not limited thereto as long as the sulfur-diffusion ratio in the carbon film could be found.

Moreover, the X-ray diffraction method was used for identifying materials present in the film, but a method therefore was not limited thereto as long as the materials present in the film could be identified.

INDUSTRIAL APPLICABILITY

The present invention is applied to a film formed on the surface of an iron-based material, and to a method for manufacturing the film.

REFERENCE SIGNS LIST

-   1: film -   10: diffusion layer -   20: nitrogen compound layer -   30: sulfurized layer -   40: carbon film -   41: hard amorphous carbon -   42: nano-carbon -   43: fullerene -   100: initial film -   110: diffusion layer -   120: nitrogen compound layer -   130: sulfurized layer -   140: carbon film -   141: hard amorphous carbon -   142: nano-carbon -   143: fullerene 

1-4. (canceled)
 5. A method for manufacturing a film formed on a molding surface of a die used for casting, comprising: an initial film-forming step for forming, on the molding surface of the die, an initial film including a carbon film, a nitrogen compound layer and a sulfurized layer, the carbon film having a plurality of nano-carbons, and a plurality of fullerenes which is applied to the carbon film, the nitrogen compound layer and the sulfurized layer being situated between the carbon film and the die; and an intermittent heating step for intermittently heating the initial film formed in the initial film-forming step under a non-oxidizing atmosphere, wherein in the intermittent heating step, an oil-based release agent is applied to the molding surface of the die on which the initial film is formed, and then the casting is performed a plurality of times using the die.
 6. A film which includes a carbon film having a plurality of nano-carbons, and which is formed on a surface of an iron-based material, characterized by containing hard amorphous carbon, Fe₄N, Fe₃C, martensite, and Fe₃O₄, and sulfur-diffusion ratio in the carbon film which is over 50%.
 7. The film according to claim 6, wherein said hard amorphous carbon, Fe₄N, Fe₃C, martensite, and Fe₃O₄ are identified by an X-ray diffraction method, and the sulfur-diffusion ratio is found by mapping analysis with an EPMA. 